The present invention relates generally to the problems associated with sectorization of cells in cellular communications systems, and more particularly to interference between cells.
Cellular radio systems provide telecommunications services to mobile users. These mobile users move through the cellular system, so-called because the geographic area of the system is divided into xe2x80x9ccellsxe2x80x9d which have base stations which are responsible for communicating with the mobile users of the system.
Each cellular system is allocated a certain bandwidth of frequencies which are available for communication with the mobile users. The available frequencies are divided between the cells so that certain frequency resources are reused by certain cells so that the distance between the reused frequencies is such that co-channel interference is maintained at tolerable levels. With smaller reuse distances a higher capacity can be provided by a cellular network. The frequency reuse distances are limited on the downlink by interference levels from the co-channel cells using the same frequencies. Typically the downlink (transmitting from a base station to a mobile station) is the limiting link. When the system is noise limited the uplink can be limiting.
In order to meet the increasing demand, numerous methods have been developed to decrease interference, thereby increasing the capacity of a cellular system. One of the most well-known solutions is to divide the covered area into sectors thereby increasing the capacity of a cellular system. In cellular systems, antennas are provided at each base station site to provide communications to the mobile systems in a given area of the system. Each base station has a plurality of sectored antennas to provide communications on a plurality of frequencies. Antennas cover an arc of e.g. 60xc2x0 or 120xc2x0, depending on the number of antenna arrays employed. In the GSM and D-AMPS systems sectors of 120xc2x0 are widely used, while in PDC systems sectors of 60xc2x0 are typical.
A key disadvantage of the sectorized approach is that radio transceivers at each cell are dedicated to particular sectors which leads to significant levels of trunking inefficiency. In practice this means that more transceivers will be needed at each base station site than for an omni-directional cell of the same capacity. In addition, each sector of the cell is treated by the cellular system as a separate cell. This means that as mobile users of the system move from one sector to another there will be considerable interaction required to handover the call to another sector, requiring higher network overhead and reducing capacity.
Another disadvantage of the sectorization approach is that it also involves an increase in hardware complexity since the transceivers are dedicated to one sector and are then not available for other sectors. This presents a major problem with present sectorization methods and techniques in their lack of flexibility in hardware allocation. If the traffic at one specific time is high in bne sector and low in another sector then it is not possible to use the transceivers in the low traffic sector cells to increase the capacity in the high traffic cells.
A broadcast control channel, or beacon, is a fundamental element in all cellular radio systems. Each sector/cell has a single broadcast channel that is assigned to a single frequency, i.e. beacon frequency, and is transmitted from the base station. The broadcast channel is used to identify the base station to the mobile users as a primary station in the area or as an additional channel to the one currently in use for e.g. handover. It also provides synchronisation and general system information to the mobile stations in the system. Each mobile station makes measurements of the signal on some or all of the broadcast channels it can receive. Results of these measurements can be used to change base stations or sectors.
It is the broadcast channel that is used to tell the mobile station which frequency to use to contact the base station and which is used when the base station is receiving an incoming call from a mobile station. Protocols in current cellular radio systems require the broadcast channel to be transmitted continuously over the entirety of the cell. In a sectored system as widely used in e.g. WCDMA, GSM or D-AMPS the beacon frequency is also used to define sector borders. In these systems a frequency has to be allocated for this purpose in each sector. This involves repeating the control part of the beacon signal for each sector, thereby occupying n-times more frequency resources for an n-sector site as compare to an omni-direction site. Furthermore, beacon frequencies are restricted to low spectral efficiency and it is not possible to use features like power control or discontinuous transmission on beacon frequencies.
Existing systems which use narrow beams to contact mobile units must then use multiple narrow beams to cover all the sectors in a cell. Existing narrow-beam systems can produce an omni-directional broadcast channel in different ways. One solution is provided where all of its narrow beams can be transmitted simultaneously. This, however, results in phase problems, not only with the base station but also with neighbouring base stations.
Another solution is provided by using an additional omni-directional antenna. The problem with this approach is that the omni-directional antenna has a significantly lower gain than a narrow beam antenna. To cover the same range as the traffic channels the omni-directional antenna requires a higher strength power amplifier.
Another prior art method is the method of having a so-called floating transceiver in a sectored site, as disclosed in AU-9475006. The floating transceiver can be switched between the different sectors and cells covered by the site, depending on the needed traffic capacity. The floating transceiver can be allocated to different sectors/cells originating from the same base station site, instead of using only one cell identity. The technique disclosed in AU-9475006 does not solve the trunking of the beacon carriers and does not involve a number of fixed transceivers, which can not be switched between the different sectors.
Yet another method is the method of having a base station antenna arrangement with a plurality of antenna apertures, as shown in EP 0795257. There are provided a plurality of beams where the traffic and broadcast channels share the same apertures and selection means are provided that select which narrow beam on which to transmit the broadcast channel. However, the invention according to this patent still has the disadvantage that the broadcast channel will only be received by a mobile station for a proportion of the time which will lead to other problems. Another problem here is that the solution shown only receives signals from a given mobile one beam at a time, rather than from all beams in the cell. This can cause problems in e.g. random access. Furthermore, the transmission signal is amplified after combination which is not possible in certain current standards, e.g. GSM.
Another state-of-the-art technique is to use adaptive antenna arrays, as disclosed in WO 95/09490. An adaptive antenna consists of an array of spatially distributed antennas. Signals are received from mobile users by the array. These are combined to extract the individual signals from the received superposition, even if they occupy the same frequency band. It is then possible to distinguish between spatially separated users by using narrow adaptive antenna lobes. The use of these narrow adaptive antenna lobes requires that the position or, more exactly, the best spatial filters for reception and/or transmission to and from, the mobile station be known. The solution provided in this patent uses a wider antenna lobe for transmitting important information on the broadcast channel. This technique implies that the signal is transmitted in a beam in the most feasible direction to the mobile station. By this technique of using adaptive antennas the transmitted interference can be even more reduced than in the case of sectorization.
However, the crucial issue of providing a matched coverage in the cell with a broadside control signal(beacon) is not addressed in state-of-the-art adaptive antennas. Furthermore, state-of-the-art use of adaptive antennas often involve architectures of digital beamforming, involving a high degree of hardware complexity, i.e. linear amplifiers, calibration, etc., thereby leading to higher costs.
The problems to be solved can be summarised as making available deployed transceiver hardware to all sectors in a cell while, at the same time, maintaining or even improving the interference situation. Furthermore, the necessity of redundant beacon channels is a problem which is avoided.
The present invention relates generally to the problems associated with sectorization of cells in cellular communications systems, and more particularly to the problems discussed above. The means of solving these problems according to the present invention are summarised in the following.
As can be seen above, there still exists a problem with current methods of dealing with interference in cellular radio systems which employ sectorization such as e.g. GSM and D-AMPS. These systems must broadcast their beacon signal over all the sectors in a cell. Broadcasting the signal over all the sectors simultaneously results in phase problems. Use of an omni-directional antenna suffers from the problems of significantly lower gain. The method of floating transceivers does not solve the trunking problems and has greater hardware complexity.
Accordingly, it is an object of the present invention to provide a method to decrease the co-channel interference in sectorized systems while decreasing the hardware complexity.
The invention provides the possibility to flexibly distribute deployed hardware between the sectors at a given cell site. This can be advantageous when the traffic is distributed differently on different hours of the day, since all transceiver hardware has access to 360 degrees coverage. At the same time, interference reduction is improved, providing a smooth migration from omni-directional sites to sectored sites.
Redundant use of the beacon carriers is also avoided. In the normal three-sectorization you need 3 beacon carriers whereas as in adaptive sectorization according to the present invention it is only necessary to deploy one carrier for the beacon for each cell rather than for each sector.
Furthermore, it is possible to combine different antenna patterns by shaping the coverage provided to the site to any general desired coverage patterns.
Downlink diversity is also obtainable by having overlapping uncorrelated antenna patterns.
Although the invention has been summarised above, the method according to the present invention is defined according to appended claims 1. Various embodiments are further defined in dependent claims 2-17.